1. Field of the Invention
This invention concerns a digital correlator or structurator in which a mathematical operation is carried out between non-delayed and delayed data. Such a correlator may perform an auto or cross correlation function.
2. Discussion of Prior Art
In a typical digital correlator an input signal is divided into successive sample intervals. Digital numbers representative of the signal during each sample interval are clocked serially through the correlator for correlation. A series of digital numbers is clocked along an M-stage shift register to form a delayed signal. A non-delayed series of digital numbers is applied to one input of M different multipliers. The other input of these multipliers is taken from successive stages of the shift register. Each multiplier thus operates on a different delay. The output of each multiplier is accumulated in one of M different counters providing M different channels. At the end of an experimental run the collective content of the counters represents the correlation function of the input signal or signals. This correlator may be termeed a linear correlator because the delay between successive channels increaases linearly.
The correlator may perform an auto correlation or a cross corrrelation on data. For an auto correlation the input signal is replicated into two identical signals; one signal is delayed and multiplied by the other non-delayed signal. For a cross correlator a first signal is delayed and multiplied with second, but non delayed, signal.
Digital signal processing enables highly accurate mathematical operations to be carried out on signals. Due to recent advances in logic speed, complicated processing can be carried out in real time. Also advances in the statistical theories of some events have simplified the processing of some functions. One example of this is in laser light scattering experiments, particularly in weak scattering events.
Detailed investigation into the properties of light scattering led to the development of a single clipped digital correlator described in U.S. Pat. No. 3,842,252. This correlator allowed the processing of signals representing the arrival of single photons on a sensitive detector. From this a whole range of work has been made possible, for example, laser light scattering where the light scattered by a suspension of particles in a liquid can be processed to give particle diffusion co-efficients.
In the above linear digital correlator a correlation function is accumulated from information obtained in successive channels. Increasing the number of sample channels allows further information to be obtained but results in increased equipment costs.
One solution to the problem of collecting information from many sample intervals is described in U.S. Pat. No. 4,593,378. In this specification the time delay between each channel is geometrically increased. Thus for example using 26 channels with a .sqroot.2 progression between channels information can be obtained from the equivalent of a delay interval of 8192 in a linear correlator e.g. U.S. Pat. No. 3,842,252. The correlator of U.S. Pat. No. 4,593,378 relies on correlating a signal, at geometrically increasing delays, to obtain the maximum information for a given number of channels. However information is still available from the non-correlated delays. Such uncollected information becomes more important at low counting rates. U.S. Pat. No. 4,593,378 also applies the principle of increasing delays to a measurement of the structure function.
Another correlator is described in U.S. Pat. No. 4,809,210; it extends the data collected from a correlator or structurator such as taught by U.S. Pat. No. 4,593,378, without the increases in equipment channels that would be involved if the linear correlator of U.S. Pat. No. 3,842,252 were merely extended. In U.S. Pat. No. 4,809,210 both the sample time interval and delay time increase in successive channels.